The present invention relates generally to the production of hydrogen chlorinated fluorocarbons (HCFC's), and more specifically to a method for preparing 1,1-dichloro-l-fluoroethane also known in the trade as HCFC-141b or R-141b.
Because of the celebrated ozone-depleting properties of chlorinated fluorocarbons (CFC's), they are falling out of favor for such uses as solvents for cleaning circuit boards, blowing agents for the extrusion of polymer foams, and aerosol propellants. Indeed, international treaties have established strict schedules for phasing out the use of CFC's.
HCFC's have been found to exhibit a relatively low ozone depletion potential, and have therefore been offered as a significant alternative to the use of CFC's. They include HCFC-141b, as well as the closely related 1,1-difluoro-1chloroethane (a.k.a., HCFC-142b) and 1,1,1-trifluoroethane (a.k.a., HFC-143a).
HCFC-141b has been prepared by a number of known methods. For example, 1,1,1-trichloroethane may be reacted with hydrogen fluoride as follows: ##STR1## to induce a halogen exchange between the chlorine and fluorine anions. U.S. Pat. No. 3,833,676 issued to Rokuo Ukaji et al. discloses such a reaction without the use of a catalyst, while U.S. Pat. No. 4,091,043 issued to Ohsaka et al., and European Published Application No. 353,059 issued to E. I. dupont de Nemours teach the use of metal halide catalysts, including antimony pentachloride. However, this halogen exchange reaction sequence suffers from several significant problems. First, for each molecule of HCFC-141b produced, a corresponding molecule of HCl is generated, which must be recovered and disposed of. Second, the trichloromethyl group of 1,1,1-trichloroethane reacts so readily in the halogen exchange reaction that multiple fluorination almost always occurs under normal operating conditions, thereby producing: ##STR2## at the expense of the desired HCFC-141b product. This reduces significantly the yield of HCFC-141b, and poses the need to separate the unwanted HCFC-142b and HFC-143a from HCFC-141b.
While E. T. McBee et al, "Fluorinated Derivatives of Ethane," Industrial and Engineering Chemistry (March 1947), pp. 409-12 is directed to a method of producing HFC-143a from either 1,1,1-trichloroethane or vinylidene chloride, the article also indicates that large amounts of HCFC-142b can be generated at high reaction temperatures and times as a side product. However, it also shows that no HCFC-141b was isolated. It is known, however, that vinylidene chloride will react directly with HF to produce HCFC-141b without the generation of Hcl, as follows: ##STR3## For example, U.K. Patent No. 627,773 issued to Chapman used stannic chloride at 35.degree. C. for 1.75 hours to catalytically induce the reaction sequence, and convert 32.7% of the vinylidene chloride to HCFC-141b. A. L. Henne et al., "The Addition of Hydrogen Fluoride to Halo-Olefins," Journal of American Chemical Society (1943), vol. 65, pp. 1271-72, discloses a reaction at 65.degree. C. for 3 hours, using 4 moles of HF without a catalyst to yield a product comprising 50% HCFC-141b, a trace of HCFC-142b, 10% unreacted vinylidene chloride, 5% CH.sub.3 CCl.sub.3 (a.k.a., R-140), and 15% tar. Twenty percent of the product stream composition is unaccounted for. These procedures, however, give poor to moderate selectivity and conversion to HCFC-141b with relatively large amounts of tar. Indeed, the propensity of vinylidene chloride to dimerize and polymerize, as well as to over-fluorinate as in the halogen exchange mechanism, makes this route appear unattractive.
Efforts have also been made with varying degrees of success to use a vapor phase process for reacting vinylidene chloride with HF to produce HCFC-141b. U.S. Pat. No. 3,755,477 issued to Firth et al. discloses the use of a steam-treated chromium oxide catalyst at 80.degree. C. to yield 46% HCFC-141b, the remainder being unwanted fluorinated products like HCFC-142b and HFC-143a. At 90.degree.-100.degree. C., however, no HCFC-141b was produced. U.S. Pat. No. 3,803,241 issued to Stolkin used alumina impregnated with a chromium salt solution at 198.degree. C. to catalytically induce a vapor-phase reaction producing 98.8% HFC-143a and 0.2% each of HCFC-141b and HCFC-142b. By contrast, European Published Application No. 353,059 issued to E.I. dupont de Nemours teaches a process passing the reagents mixed in the vapor phase through an aluminum fluoride catalyst at 74.degree.-86.degree. C. using a molar HF/vinylidene chloride ratio of 4.3 to produce a product stream comprising 99.8% HCFC-141b, 0.1% HCFC-142b, and 0.1% unreacted vinylidene chloride with an 89.6% yield. The missing 10% of the product stream is probably tar, which would shorten the life of the catalyst.
Vinylidene chloride has also been reacted with HF in the liquid phase to produce HCFC-142b, as disclosed by Japanese Published Application No. 47-39086 issued to Kureha Kagaku Kogyo Co., Ltd., using a stannous chloride catalyst. Running the process at 90.degree. C. for 60 minutes using a 6.0 HF/vinylidene chloride ratio, 96.4% of the vinylidene chloride reagent was converted to fluorochloroethanes, for a yield of 76.4% HCFC-142b, 8.0% HCFC-141b, and 12.0% HFC-143a--a high conversion rate, but an exceedingly low yield of HCFC-141b product. The disclosure also indicates that when TiCl.sub.4 catalyst was used, 40.4% of the resulting product stream comprised HCFC-141b, while HCFC-142b accounted for 4.0%. However, identifiable organic products only accounted for 51% of the product stream leaving 49% for tar. The moderate yield of HCFC-141b product and large production of tar makes this process undesirable.